Efficiency in boiler operations is serious business, and rightfully so. A coal steam boiler typically consumes 20 times the value of its capital investment in the cost of fuel during it economic lifespan. So if one spends any amount of effort acquiring the right boiler for the job, one should by comparison spend 20 times as much effort in ensuring that every bit of fuel produces its maximum contribution to the process of steam generation. Considering too that the basic design of the packaged fire tube boiler has not undergone any significant change since the times when coal was generally plentiful, cheap and of excellent quality, making it so much harder for steam users to improve efficiency with odds stacked against their reasonable efforts.
It is therefore clear that achieving high boiler plant efficiency is no stroke of luck or something that falls into one’s lap. It takes hard work, understanding, dedication and excellent management to continuously operate a steam boiler at peak efficiency. And all for one objective: to produce steam at the lowest cost per unit. But also understanding that assets must be preserved and the carbon footprint minimized in pursuit of peak efficiency.
So let us jump straight into some of the definitions pertaining to “efficiency”, and there is a multitude of different interpretations of it; one can easily get totally confused. But in essence efficiency expresses as a percentage the ratio of useful energy output from a system to the total energy input into the system. In a boiler environment the energy input comes from the fuel, the energy output is found in the energy added to the produced steam.
In a perfect system without any energy loss the efficiency will be 100%. Unfortunately we live in a broken world and energy loss in any thermal system is a reality. The real challenge is minimizing the energy loss from the system on a continuous basis, remembering always that energy once lost from the system cannot be recovered!
Our first challenge is to clarify the value of energy input per unit of fuel. Some schools prefer gross calorific value (GCV, also called higher heating value), others net calorific value (NCV, also called lower heating value). The difference between the two is the latent heat of water vapour formed during combustion. Thus NCV = GCV – Latent Heat. Using GCV renders a lower efficiency percentage than when NCV is used. So next time you encounter boiler efficiency statistics and comparisons, clarify beforehand if it is based on GCV or NCV. It can make a significant difference, particularly with oil and gas fired boilers where the hydrogen content of the fuel is relatively high. With coal fired boilers it is customary to use GCV in efficiency calculations; with oil and gas fired boilers it is NCV.
To complicate matters further efficiency can be calculated in two different ways; according to the direct method or to the indirect method.
The direct method is based on energy added to the steam as a percentage of the fuel energy input. The equation below renders the system (or overall or thermal) efficiency (not the boiler efficiency!), where
- The added energy is calculated as (Steam
energy – energy of feed water), and
- Steam energy = Steam Flow * Saturated steam enthalpy (the latter available from steam tables).
- Feed water energy = Steam Flow * Feed water enthalpy, (also available from steam tables, or calculated = 4.2 * feed water temperature deg. C).
- The fuel input energy = Fuel feed * Calorific value.
- System efficiency % = Added energy * 100 / Fuel energy.
- Now note that the direct method incorporates all energy losses in the system, including those encountered in the steam and condensate reticulation systems and in the production facility where the steam is used. This is the most practical and popular way of calculating system efficiency, as all of the required parameters to do the calculation are normally readily available. The equation is often simplified by substituting feed water flow for steam flow, which means blow down quantity is ignored. The bigger challenge is probably to accurately measure steam and fuel usages.
- Physical steam and condensate losses are reflected in the make-up water quantity, while energy losses are reflected in the temperature of the feed water to the boiler (without any steam being added).
The indirect method starts off with 100% of fuel energy input and then deducts individual calculated or estimated energy losses encountered within the combustion process, such as stack loss, carbon loss, blow down loss, shell loss, etc. The indirect method calculates energy loss across the boiler only and normally fails to address energy loss downstream of the boiler, i.e. along steam and condensate lines and in steam application processes. These can however be approximated if the make-up water percentage and temperature is known, since make-up water replenishes downstream steam and condensate losses. I use this approach in my combustion calculator; not absolutely accurate, but sufficiently useful for purposes of evaluating boiler performance under various operating conditions, or with coal of differing characteristics.
In the Boiler Bits 9 issue we have identified the following significant energy losses from the boiler:
- Stack loss, consisting of dry heat loss, combustibles loss and wet loss.
- Shell loss, consisting of radiation and convection losses.
- Bottom ash loss
- Blow down loss
By definition then:
- Combustion Efficiency % = (100% – Stack loss %)
- Boiler Efficiency % = (Combustion Efficiency % – Shell Loss % – Bottom ash loss % – Blow down Loss %*)
- System Efficiency % = (Boiler Efficiency % – Reticulation and Process Loss %).
* Some schools classify blow down loss as part of system loss.
One may encounter foreign terminology to express steam plant efficiency, or certain aspects thereof. In all instances make sure a clear understanding is gained of how this particular efficiency is defined and calculated; particularly if efficiencies are compared between boilers as part of a new boiler acquisition investigation. It is absolutely essential that one compares apples with apples in all instances.
In the next issue of Boiler Bits I will be discussing what levels of efficiency are considered top notch and achievable on a sustainable basis, specifically with chain grate fire tube boilers.
This post was compiled by René le Roux for Le Roux Combustion, all rights reserved. Do you want to know more about combustion control systems and combustion optimization? Please contact us for your professional boiler automation, steam system efficiency and coal characterization needs.
Kindly note that our posts do not constitute professional advice and the comments, opinions and conclusions drawn from this post must be evaluated and implemented with discretion by our readers at their own risk.